Devices and methods for video coding

ABSTRACT

A video coding block is intra-predicted on the basis of reference samples from a set of neighboring video coding blocks. A fitting plane is determined on the basis of the reference samples. The fitting plane defines a plurality of fitting samples. For a selected directional intra prediction mode, the sample values of the video coding block are predicted on the basis of the reference sample values and the fitting sample values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/RU2017/000491, filed on Jul. 5, 2017, the disclosure of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments of the invention relate to the field of video coding. Morespecifically, the embodiments of the invention relate to an apparatusand a method for intra prediction of a video coding block as well as anencoding apparatus and a decoding apparatus comprising such anintra-prediction apparatus.

BACKGROUND

Digital video communication and storage applications are implemented bya wide range of digital devices, e.g. digital cameras, cellular radiotelephones, laptops, broadcasting systems, video teleconferencingsystems, etc. One of the most important and challenging tasks of theseapplications is video compression. The task of video compression iscomplex and is constrained by two contradicting parameters: compressionefficiency and computational complexity. Video coding standards, such asITU-T H.264/AVC or ITU-T H.265/HIEVC, provide a good tradeoff betweenthese parameters. For that reason support of video coding standards is amandatory requirement for almost any video compression application.

The state-of-the-art video coding standards are based on partitioning ofa source picture into video coding blocks. Processing of these blocksdepend on their size, spatial position and a coding mode specified by anencoder. Coding modes can be classified into two groups according to thetype of prediction: intra- and inter-prediction modes. Intra-predictionmodes use pixels of the same picture (also referred to as frame orimage) to generate reference samples to calculate the prediction valuesfor the pixels of the block being reconstructed. Intra-prediction isalso referred to as spatial prediction. Inter-prediction modes aredesigned for temporal prediction and uses reference samples of previousor next pictures to predict pixels of the block of the current picture.After a prediction stage, transform coding is performed for a predictionerror that is the difference between an original signal and itsprediction. Then, the transform coefficients and side information areencoded using an entropy coder (e.g., CABAC for AVC/H.264 andHEVC/H.265). The recently adopted ITU-T H.265/HEVC standard (ISO/IEC23008-2:2013, “Information technology—High efficiency coding and mediadelivery in heterogeneous environments—Part 2: High efficiency videocoding”, November 2013) declares a set of state-of-the-art video codingtools that provide a reasonable tradeoff between coding efficiency andcomputational complexity. An overview on the ITU-T H.265/HEVC standardhas been given by Gary J. Sullivan, “Overview of the High EfficiencyVideo Coding (HEVC) Standard”, in IEEE Transactions on Circuits andSystems for Video Technology, Vol. 22, No. 12, December 2012, the entirecontent of which is incorporated herein by reference.

Similarly to the ITU-T H.264/AVC video coding standard, the HEVC/H.265video coding standard provides for a division of the source picture intoblocks, e.g., coding units (CUs). Each of the CUs can be further splitinto either smaller CUs or prediction units (PUs). A PU can be intra- orinter-predicted according to the type of processing applied for thepixels of PU. In case of inter-prediction, a PU represents an area ofpixels that is processed by motion compensation using a motion vectorspecified for a PU. For intra prediction, the adjacent pixels ofneighbor blocks are used as reference samples to predict a currentblock. A PU specifies a prediction mode that is selected from the set ofintra-prediction modes for all the transform units (TUs) contained inthis PU. A TU can have different sizes (e.g., 4×4, 8×8, 16×16 and 32×32pixels) and can be processed in different ways. For a TU, transformcoding is performed, i.e. the prediction error is transformed with adiscrete cosine transform or a discrete sine transform (in theHEVC/H.265 standard, it is applied to intra-coded blocks) and quantized.Hence, reconstructed pixels contain quantization noise (it can becomeapparent, for examples, as blockiness between units, ringing artifactsalong with sharp edges, etc.) that in-loop filters such as DeblockingFilter (DBF), Sample Adaptive Offset (SAO) and Adaptive Loop Filter(ALF) try to suppress. The use of sophisticated prediction coding (suchas motion compensation and intra-prediction) and partitioning techniques(e.g., quadtree for CUs and PUs as well as residual quadtree for TUs inthe HEVC/H.265 standard and quadtree plus binary tree for the JEMreference software from version JEM-3.0 onwards) allowed thestandardization committee to significantly reduce the redundancy in PUs.The fundamental difference between the quadtree (QT) and quadtree plusbinary tree (QTBT) partitioning mechanisms is that the latter oneenables not only square but also rectangular blocks by usingpartitioning based on both quad- and binary-tree.

In the H.264/AVC standard, four intra-prediction modes were availablefor 16×16 blocks for a luma color component. One of those modes isplane-based and can predict a source-signal gradient within a block. Theformula used to calculate pixels to be predicted using the plane-basedmode is expressed as follows:p _(pred)[x,y]=clip3(0,2^(n)−1,(a+b(x−7)+c(y−7)+16)>>5),where a, b and c are plane (multiple regression) parameters. It is worthnoting that the clip3 function, p_(pred)[x,y]=clip 3(p_(min), p_(max),{circumflex over (p)}_(pred)[x,y]), is used in the equation above. Inthe clip3 function, p_(min) and p_(max) are the minimum and maximumvalues of pixels that are possible for a given bit depth (e.g.,p_(min)=0 and p_(max)=255 for bit depth 8) respectively; {circumflexover (p)}_(pred)[x,y] and p_(pred)[x,y] are values of predictors at theposition [x,y] before and after clipping respectively.

According to the HEVC/H.265 standard, 35 intra prediction modes areavailable and include a planar mode (the intra-prediction mode index is0), DC mode (the intra-prediction mode index is 1), and 33 directionalmodes (the intra-prediction mode index ranges from 2 to 34). From theJEM-1.0 software onwards, the set of directional intra-prediction modeshas been extended up to 65 modes (almost doubled) by decreasing a stepangle between directional intra-prediction modes by a factor of 2. Asseen from the listed modes above, the plane-based mode was adoptedneither for HEVC/H.265 nor for the JEM software. In fact, this mode wasreplaced by the planar one that does not always result in a plane-basedpredictor.

As discussed in “EE7 Adaptive Clipping in JEM3.0” by F. Galpin et al.,Contribution JVET-D0033 to the 4^(th) JVET meeting, China, 2016, theadaptive clipping mechanism, initially proposed in “Adaptive Clipping inJEM2.0” by F. Galpin et al., Contribution JVET-C0040 to the 3^(rd) JVETmeeting, Switzerland, 2016, is used to restrict pixel values in blocks(e.g., in a predictor) from the JEM-4.0 software onwards. This techniqueuses clipping bounds that are determined at the encoder side and areexplicitly signaled in the bit-stream, namely, in slice headers.Clipping bounds are defined as actual minimum p_(min)(C) and maximump_(max)(C) sample values of coded pictures separately for every colorcomponent. Mathematically, adaptive clipping operation can be presentedas follows:p _(pred)(x,y,C)=clip3(p _(min)(C),p _(max)(C),{circumflex over (p)}_(pred)(x,Y,C))=clip A({circumflex over (p)} _(pred)(x,Y,C),C),where C is an index of a selected color component. This mechanism,similar to the clip 3( ) function, is directly applied to pixel values,e.g., within a predictor.

Prediction of a block may include steps of generating secondaryreference samples that are located on the sides of the block that arenot yet reconstructed and to be predicted, i.e. unknown pixels. Valuesof these secondary reference samples are derived from the primaryreference samples which are obtained from the pixels of the previouslyreconstructed part of the picture, i.e., known pixels. Evidently, theblock that is decoded first within a picture cannot be predicted usingpreviously reconstructed pixels. The same situation occurs when a sideof the block to be predicted is out of the picture boundary. In thissituation primary reference samples are generated using a pre-definedset of rules. In case of H.265/HEVC standard, they are assigned to avalue equal to half of the possible maximal value of a pixel that isdetermined by a bit depth of the corresponding picture color plane.There are different methods that generate secondary reference samplesfor predicting a block. Since values of the secondary reference samplesdepend on the primary reference samples, the step of calculating themcould be performed implicitly when prediction of a pixel value iscalculated. The planar intra-prediction mode of H.265/HEVC standard isan example of such a method.

However, the plane-based intra prediction mode has a major problem: theregression (plane) parameters are being determined without taking intoaccount constraints caused by actual pixel values. Such pixel values arepossible to provide a given bit depth or actual minimum and maximumsample values of coded pictures or minimum and maximum values ofreference samples used for constructing the plane. Therefore, thecurrent plane-based mode for intra-prediction suffers from lowcompression efficiency.

In light of the above, there is a need for improved devices and methodsfor video coding, which allow increasing the coding efficiency for intraprediction.

SUMMARY

It is an object of the invention to provide improved devices and methodsfor video coding, which allow increasing the coding efficiency for intraprediction.

The foregoing and other objects are achieved by the systems and methodsas set forth in the description that follows, which are provided by wayof example and not limitation. Further implementation forms will beapparent from the description and the figures.

Generally, embodiments of the invention relate to hybrid video coding,in particular intra-prediction. More specifically, embodiments of theinvention provide an improvement of intra-prediction by combining anydirectional intra-prediction mechanism with the plane-basedintra-prediction mode (with or without clipping of plane parameters),which provides better coding efficiency than conventional plane-basedand directional intra-prediction modes. According to embodiments of theinvention, both directional elements and gradient changes within a blockcan be predicted. Two example implementations of the intra-predictionmethod according to embodiments of the invention will be presented indetail further below, wherein clipping can be applied to both regression(plane) parameters and pixel values within a predictor in contrast tothe conventional plane-based mode, wherein only values of pixels withina predictor are clipped.

In contrast to conventional approaches embodiments of the inventionallow taking into account that the intensities of propagated (orinterpolated) reference samples can change along a selectedintra-prediction direction. The intensity change of the referencesamples depends on the incline of a plane fitted to primary referencesamples. A prediction direction is selected independently of planeparameters.

Thus, embodiments of the invention provide in particular the followingadvantages: firstly, additional gain of coding can be reached byintegrating embodiments of the invention into a codec. Secondly,embodiments of the invention can be used in many potential applicationsin hybrid video coding paradigms that are compatible with the HMsoftware and the VPX video codec family as well as the JEM software andthe VPX/AV1 video codec family, i.e., a state-of-the-art andnext-generation video coding frameworks, respectively. Thirdly, hardwareand computational complexities can be kept low at both encoder anddecoder sides. Finally, embodiments of the invention can be easilyimplemented in codecs that use conventional intra-prediction mechanisms.

The following disclosure employs a plurality of terms which, inembodiments, have the following meaning: Slice—a spatially distinctregion of a picture that is independently encoded/decoded. Sliceheader—Data structure configured to signal information associated with aparticular slice. Video coding block (or short block)—an M×N (M-columnby N-row) array of pixels or samples (each pixel/sample being associatedwith at least one pixel/sample value), or an M×N array of transformcoefficients. Coding Tree Unit (CTU) grid—a grid structure employed topartition blocks of pixels into macro-blocks for video encoding. CodingUnit (CU)—a coding block of luma samples, two corresponding codingblocks of chroma samples of an image that has three sample arrays, or acoding block of samples of a monochrome picture or a picture that iscoded using three separate color planes and syntax used to code thesamples. Picture Parameter Set (PPS)—a syntax structure containingsyntax elements that apply to zero or more entire coded pictures asdetermined by a syntax element found in each slice segment header.Sequence Parameter Set (SPS)—a syntax structure containing syntaxelements that apply to zero or more entire coded video sequences asdetermined by the content of a syntax element found in the PPS referredto by a syntax element found in each slice segment header. VideoParameter Set (VPS)—a syntax structure containing syntax elements thatapply to zero or more entire coded video sequences. Prediction Unit(PU)—a prediction block of luma samples, two corresponding predictionblocks of chroma samples of a picture that has three sample arrays, or aprediction block of samples of a monochrome picture or a picture that iscoded using three separate color planes and syntax used to predict theprediction block samples. Transform Unit (TU)—a transform block of lumasamples, two corresponding transform blocks of chroma samples of apicture that has three sample arrays, or a transform block of samples ofa monochrome picture or a picture that is coded using three separatecolor planes and syntax used to predict the transform block samples.Supplemental enhancement information (SEI)—extra information that may beinserted into a video bit-stream to enhance the use of the video.Luma—information indicating the brightness of an image sample.Chroma—information indicating the color of an image sample, which may bedescribed in terms of red difference chroma component (Cr) and bluedifference chroma component (Cb).

More specifically, according to a first aspect the invention relates toan apparatus for intra prediction of a current video coding block of aframe of a video signal on the basis of a plurality of reference samplesfrom a set of neighboring video coding blocks of the current videocoding block, the current video coding block comprising a plurality ofsamples, each sample being associated with a sample value and a positionwithin the frame. The apparatus comprises a processing unit configuredto: determine a fitting plane on the basis of the plurality of referencesamples, wherein the fitting plane defines a plurality of fittingsamples, each fitting sample being associated with a fitting samplevalue and a position within the frame; and predict for a selecteddirectional intra prediction mode the sample values of the plurality ofsamples of the current video coding block on the basis of the pluralityof reference sample values, the plurality of fitting sample valueswithin the current block and at the positions of reference samples.

Thus, an improved apparatus for video coding is provided, which allowsincreasing the coding efficiency for intra prediction.

In a further example implementation form of the first aspect, theprocessing unit is configured to determine the fitting plane on thebasis of the plurality of reference samples by determining fitting planeparameters a, b and c on the basis of the plurality of reference samplessuch that the plurality of fitting sample values {circumflex over(p)}_(pred)[x,y] are defined by the following equation:{circumflex over (p)} _(pred)[x,y]=ax+by+c,wherein x, y denote the position of the fitting sample within the frame.

In a further example implementation form of the first aspect, theprocessing unit is configured to perform a multi-linear regression, inparticular a least squares method, for determining the fitting planeparameters on the basis of the plurality of reference samples.

In a further example implementation form of the first aspect, theprocessing unit is further configured to clip the respective fittingplane parameters a, b and/or c, in case the respective fitting planeparameters a, b and/or c lie outside of respective predefined allowableranges of fitting plane parameters.

In a further example implementation form of the first aspect, forpredicting for the selected directional intra prediction mode the samplevalues of the plurality of samples of the current video coding block onthe basis of the plurality of reference sample values and the pluralityof fitting sample values, the processing unit is configured to:determine for each reference sample a respective reference offset valuebetween the reference sample value and the corresponding fitting samplevalue; determine for a respective sample value of the plurality ofsamples of the current video coding block a respective samplepropagated/interpolated offset value on the basis of at least a subsetof the plurality of reference offset values and the selected directionalintra prediction mode; and generate a respective sample value on thebasis of the respective sample propagated/interpolated offset value andthe fitting sample value of the respective fitting sample.

In a further example implementation form of the first aspect, theprocessing unit is configured to generate the respective sample value onthe basis of the respective sample propagated/interpolated offset valueand the fitting sample value of the respective fitting sample by addingthe respective sample propagated/interpolated offset value to thefitting sample value of the respective fitting sample.

In a further example implementation form of the first aspect, theprocessing unit is configured to predict the respective samplepropagated/interpolated offset value at each sample position on thebasis of at least a subset of the plurality of reference offset valuesand the selected directional intra prediction mode on the basis of anintra prediction mode defined in the HEVC/H.265 standard or a standardevolved therefrom.

In a further example implementation form of the first aspect, the set ofneighboring video coding blocks of the current video coding blockcomprises a video coding block above the current video coding blockand/or a video coding block to the left of the current video codingblock.

According to a second aspect the invention relates to an encodingapparatus for encoding a current video coding block of a frame of avideo signal, the current video coding block comprising a plurality ofsamples, each sample being associated with a sample value. The encodingapparatus comprises: an intra prediction apparatus according to thefirst aspect for providing a predicted video coding block; and anencoding unit configured to encode the current video coding block on thebasis of the predicted video coding block.

Thus, an improved encoding apparatus for video coding is provided, whichallows increasing the encoding efficiency for intra prediction.

According to a third aspect the invention relates to a decodingapparatus for decoding an encoded video coding block of a frame of avideo signal, the encoded video coding block comprising a plurality ofsamples, each sample being associated with a sample value. The decodingapparatus comprises: an intra prediction apparatus according to thefirst aspect for providing a predicted video coding block; and arestoration unit configured to restore a video coding block on the basisof an encoded video coding block and the predicted video coding block.

Thus, an improved decoding apparatus for video coding is provided, whichallows increasing the decoding efficiency for intra prediction.

According to a fourth aspect the invention relates to a method for intraprediction of a current video coding block of a frame of a video signalon the basis of a plurality of reference samples from a set ofneighboring video coding blocks of the current video coding block, thecurrent video coding block comprising a plurality of samples, eachsample being associated with a sample value and a position within theframe.

The method comprises: determining a fitting plane on the basis of theplurality of reference samples, wherein the fitting plane defines aplurality of fitting samples, each fitting sample being associated witha fitting sample value and a position within the frame; and predictingfor a selected directional intra prediction mode the sample values ofthe plurality of samples of the current video coding block on the basisof the plurality of reference sample values and the plurality of fittingsample values.

Thus, an improved method for video coding is provided, which allowsincreasing the coding efficiency for intra prediction.

The intra prediction method according to the fourth aspect of theinvention can be performed by the intra prediction apparatus accordingto the first aspect of the invention. Further features of the intraprediction method according to the fourth aspect of the invention resultdirectly from the functionality of the intra prediction apparatusaccording to the first aspect of the invention and its differentimplementation forms.

According to a fifth aspect the invention relates to a computer programcomprising program code for performing the method according to thefourth aspect when executed on a computer.

Embodiments of the invention can be implemented in hardware and/orsoftware.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect tothe following figures, wherein:

FIG. 1 shows a schematic diagram illustrating an intra predictionapparatus according to an embodiment;

FIG. 2 shows a schematic diagram illustrating an encoding apparatusaccording to an embodiment and a decoding apparatus according to anembodiment;

FIG. 3 shows a schematic diagram illustrating an intra prediction methodaccording to an embodiment;

FIG. 4A shows a schematic diagram illustrating a video coding block tobe predicted by an intra prediction apparatus according to anembodiment;

FIG. 4B shows a schematic diagram illustrating a fitting planedetermined by an intra prediction apparatus according to an embodiment;

FIG. 5 shows a schematic diagram illustrating offset values determinedby an intra prediction apparatus according to an embodiment;

FIG. 6 shows a schematic diagram of a video coding block illustratingdifferent aspects of an intra prediction apparatus according to anembodiment;

FIG. 7 shows a schematic diagram of a video coding block illustratingdifferent aspects of an intra prediction apparatus according to anembodiment; and

FIG. 8 shows a schematic diagram of a video coding block illustratingdifferent aspects of an intra prediction apparatus according to anembodiment.

In the various figures, identical reference signs will be used foridentical or at least functionally equivalent features.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings, which form part of the disclosure, and in which are shown, byway of illustration, specific aspects in which embodiments of theinvention may be placed. It is understood that other aspects may beutilized and structural or logical changes may be made without departingfrom the scope of embodiments of the invention. The following detaileddescription, therefore, is not to be taken in a limiting sense, as thescope of embodiments of the invention is defined by the appended claims.

For instance, it is understood that a disclosure in connection with adescribed method may also hold true for a corresponding device or systemconfigured to perform the method and vice versa. For example, if aspecific method step is described, a corresponding device may include aunit to perform the described method step, even if such unit is notexplicitly described or illustrated in the figures. Further, it isunderstood that the features of the various exemplary aspects describedherein may be combined with each other, unless specifically notedotherwise.

FIG. 1 shows a schematic diagram illustrating an intra predictionapparatus 100 according to an embodiment.

With further reference to FIG. 4A, the intra prediction apparatus 100 isconfigured to intra predict a current video coding block 401 of a frameof a video signal on the basis of a plurality of reference samples 403from a set of neighboring video coding blocks of the current videocoding block 401, wherein the current video coding block 401 comprises aplurality of samples 405, each sample being associated with a samplevalue and a position within the frame.

As can be taken from FIG. 1, the intra prediction apparatus 100comprises a processing unit 101 configured to determine a fitting plane501 (shown in FIG. 4B) on the basis of the plurality of referencesamples 403, wherein the fitting plane 501 defines a plurality offitting samples, each fitting sample being associated with a fittingsample value and a position within the frame. The processing unit 101 isfurther configured to predict for a selected directional intraprediction mode the sample values of the plurality of samples 405 of thecurrent video coding block 401 on the basis of the plurality ofreference sample values, the plurality of fitting sample values withinthe current block 401 and at the positions of reference samples 403.

As can be taken from FIGS. 4A, 6, 7 and 8, in an embodiment, the set ofneighboring video coding blocks of the current video coding block 401comprises a video coding block above the current video coding block 401and/or a video coding block to the left of the current video codingblock 401.

Further embodiments of the intra prediction apparatus 100 will bedescribed below.

FIG. 2 shows a schematic diagram illustrating an encoding apparatus 201according to an embodiment and a decoding apparatus 211 according to anembodiment.

The encoding apparatus 201 is configured to encode a current videocoding block 401 of a frame of a video signal, wherein the current videocoding block 401 comprises a plurality of samples 405, each sample beingassociated with a sample value. The encoding apparatus 201 comprises theintra prediction apparatus 100 shown in FIG. 1 for providing a predictedvideo coding block and an encoding unit 203 configured to encode thecurrent video coding block 401 on the basis of the predicted videocoding block and providing the encoded current video coding block, forinstance, in the form of a bitstream. Further embodiments of theencoding apparatus 201 will be described further below. In anembodiment, the encoding apparatus 201 could be implemented as a hybridencoder, as defined, for instance, in the HEVC standard, and couldcomprise further components, such as an entropy encoder.

The decoding apparatus 211 is configured to decode the encoded videocoding block of a frame of a video signal, which is contained in thebitstream provided by the encoding apparatus 201, wherein the encodedvideo coding block comprises a plurality of samples, each sample beingassociated with a sample value. The decoding apparatus 211 comprises theintra prediction apparatus 100 shown in FIG. 1 for providing a predictedvideo coding block and a restoration unit 213 configured to restore avideo coding block on the basis of the encoded video coding block andthe predicted video coding block. Further embodiments of the decodingapparatus 211 will be described further below. In an embodiment, thedecoding apparatus 211 could be implemented as a hybrid decoder, asdefined, for instance, in the HEVC standard, and could comprise furthercomponents, such as a decoding unit for providing a residual videocoding block on the basis of the encoded video coding block.

FIG. 3 shows a schematic diagram illustrating a method 300 for intraprediction of a current video coding block 401 of a frame of a videosignal on the basis of a plurality of reference samples 403 from a setof neighboring video coding blocks of the current video coding block 401according to an embodiment, wherein the current video coding block 401comprises a plurality of samples 405 and each sample is associated witha sample value and a position within the frame.

The intra prediction method 300 comprises a step 301 of determining afitting plane 501 on the basis of the plurality of reference samples403, wherein the fitting plane 501 defines a plurality of fittingsamples, each fitting sample being associated with a fitting samplevalue and a position within the frame.

Moreover, the intra prediction method 300 comprises a step 303 ofpredicting for a selected directional intra prediction mode the samplevalues of the plurality of samples 405 of the current video coding block401 on the basis of the plurality of reference sample values and theplurality of fitting sample values. Further embodiments of the intraprediction method 300 will be described further below.

In a first implementation form of the invention, the intra predictionmethod 300 can be performed by the processing unit 101 of the intraprediction apparatus 100 mentioned above in FIG. 1. The detailed stepsof the first implementation form in the intra prediction apparatus 100will be discussed further below.

FIG. 4A shows a schematic diagram of an exemplary current video codingblock 401 illustrating an aspect of the intra prediction apparatus 100and the intra prediction method 300 according to an embodiment, inparticular the relationship between the reference pixels (also referredto as samples) 403 and intra predicted pixels or samples, wherein thepixel or sample of the currently processed video coding block 401, i.e.the currently processed pixel/sample, is identified by a darker shade ofgrey and the reference pixels/samples 403 are identified by squares withdots.

For the exemplary current video coding block 401 shown in FIG. 4A, thereference pixels or samples 403 are the pixels in the row of pixelsabove the current video coding block 401 and the pixels in the column ofpixels to the left of the current video coding block 401, which willalso be referred to as “primary reference pixels or samples” 403 in thefollowing. Thus, in the embodiment shown in FIG. 4A, the primaryreference pixels 403 belong to neighboring video coding blocks, whichalready have been intra predicted, i.e. processed by the intraprediction apparatus 100.

FIG. 4B shows a schematic diagram of a fitting plane 501 predicted bythe processing unit 101 using a MLR-based model, wherein primaryreference samples 403 as illustrated in FIG. 4A are used to estimateregression parameters of the MLR-based model for constructing thefitting plane 501. As a comparison, a fitting plane predicted by the DCmode in the HEVC/H.265 standard is also shown in FIG. 4B. Thus, theprocessing unit 101 is configured to estimate parameters of a MultipleLinear Regression (MLR) and fit a plane 501 to the primary referencesamples 403 (step 1).

More specifically, in an embodiment, the processing unit 101 isconfigured to determine the fitting plane 501 on the basis of theplurality of reference samples 403 by determining fitting planeparameters a, b and c on the basis of the plurality of reference samples403 such that the plurality of fitting sample values {circumflex over(p)}_(pred)[x,y] are defined by the following equation:{circumflex over (p)} _(pred)[x,y]=ax+by+c,wherein x, y denote the position of the fitting sample within the frame.

In step 1, different approaches can be used for the Multiple LinearRegression (MLR). For instance, in an embodiment the processing unit 101is configured to perform a Multi-Linear Regression, in particular aleast-squares method, for determining the fitting plane parameters onthe basis of the plurality of reference samples 403. This well-knownapplicable least-square method provides such values of regressionparameters that minimize the sum of error squares between the data usedfor estimating regression parameters (i.e. primary reference samples403) and the values calculated using the above equation at the positionscorresponding to primary reference samples 403. In fact, this step canbe similar to the plane-based mode employed in the H.264/AVC standard.

In a step 2, the processing unit 101 can optionally clip the plane(regression) parameters, as the parameters estimated in step 1 are notguaranteed to fall within a practical range. Therefore, it can be foundpractical to perform their clipping in step 2. Moreover, a pre-definedthreshold could be applied so that the parameters a, b and c will notexceed the corresponding pre-defined threshold value(s).

Thus, in an embodiment, the processing unit 101 of the intra predictionapparatus 100 is further configured to clip the respective fitting planeparameters, a, b and/or c, in case the respective fitting planeparameters a, b and/or c lie outside of respective predefined allowableranges of fitting plane parameters.

In a step 3, the processing unit 101 of the intra prediction apparatus100 is configured to use clipped plane parameters (the result from step2) to calculate gradient signal values at positions of reference samples601 (shown in FIG. 6) that are the pixels in the row of pixels below thecurrent video coding block 401 and the pixels in the column of pixels tothe right of the current video coding block 401, which will also bereferred to as “secondary reference samples” 601, {circumflex over(p)}_(srs), in the following.

In a step 4, the processing unit 101 of the intra prediction apparatus100 is further configured to use clipped plane parameters (the resultfrom step 2) to calculate gradient signal values at positions of theprimary reference samples 403, {circumflex over (p)}_(prs).

In a step 5, the processing unit 101 of the intra prediction apparatus100 is configured to subtract the result of the previous step, i.e. thegradient signal values of the primary reference samples 403, from theactual values of the primary reference samples 403, which is illustratedin FIG. 5. Thus, FIG. 5 shows a schematic diagram of an embodiment,wherein, for generating the plurality of secondary reference samples 601on the basis of the plurality of primary reference samples 403 and theplurality of fitting samples for the selected directional intraprediction mode, the processing unit 101 is configured to determine foreach primary reference sample a respective primary offset value betweenthe primary reference sample value and the corresponding fitting samplevalue, wherein the primary offset value, the primary reference samplevalue and the corresponding fitting sample value are represented bysquares with diagonal stripes, dots and vertical stripes in FIG. 5respectively.

The offset value between the primary reference sample value and thecorresponding fitting sample value can be explicitly expressed by thefollowing equation:Δ[k]=p _(prs)[k]−{circumflex over (p)} _(prs)[k]wherein p_(prs) and {circumflex over (p)}_(prs) denote the actual andgenerated primary reference sample values.

In further steps, the processing unit 101 is configured to predict arespective secondary offset value at each secondary reference sampleposition on the basis of at least a subset of the plurality of primaryoffset values and the selected intra prediction mode, and to generate arespective secondary reference sample value on the basis of therespective secondary offset value and the fitting sample value of therespective fitting sample, which are implemented as steps 6 and 7respectively and are explained with reference to FIG. 6 below, whichillustrates the relationship between primary reference pixels 403,secondary reference pixels 601 and intra predicted pixels according toembodiments of the invention.

In FIG. 6 the grey square of pixels represents the exemplary currentlyprocessed video coding block 401. For the exemplary current video codingblock 401 shown in FIG. 6, the primary reference pixels 403 p_(prs) arethe pixels in the row of pixels above the current video coding block 401and the pixels in the column of pixels to the left of the current videocoding block 401. Moreover, the secondary reference pixels 601 p_(srs)are the pixels in the row of pixels below the current video coding block401 and the pixels in the column of pixels to the right of the currentvideo coding block 401.

FIG. 6 illustrates as an example the case, where the intra predictionapparatus 100 propagates the offset value between the actual 403 andgenerated primary reference samples on the basis of a selecteddirectional intra prediction mode onto positions of secondary referencesamples 601 which are identified in FIG. 6 by squares with zigzags. Inan embodiment, the propagation can be performed by means of thedirectional intra prediction using sub-pixel interpolation mechanisms;wherein the direction of propagation is defined by the intra predictionmode defined for the block to be predicted.

FIG. 6 further illustrates as an example the case, where in step 7 theprocessing unit 101 of the intra prediction apparatus 100 is configuredto generate the respective secondary reference sample value on the basisof the respective secondary offset value and the fitting sample value ofthe respective fitting sample by adding the respective secondary offsetvalue to the fitting sample value of the respective fitting sample.

Furthermore, the processing unit 101 of the intra prediction apparatus100 is configured to predict the respective secondary propagated offsetvalue at each secondary reference sample position on the basis of atleast a subset of the plurality of primary offset values and theselected directional intra prediction mode on the basis of an intraprediction mode defined in the HEVC/H.265 standard or a standard evolvedtherefrom.

Finally, in a step 8 the processing unit 101 of the intra predictionapparatus 100 is configured to predict the sample value of the sample405 of the currently processed video coding block 401, i.e. thecurrently processed sample 405.

FIG. 7 shows an exemplary current video coding block 401, illustratingan aspect of the intra prediction apparatus 100 and the intra predictionmethod 300 according to an embodiment, in particular the relationshipbetween primary reference pixels 403, secondary reference pixels 601 andintra predicted pixels, wherein the grey square of pixels represents theexemplary currently processed video coding block 401 and the squareswith dots and zigzags denote the primary 403 and secondary 601 referencesamples respectively.

In an embodiment, the processing unit 101 of the intra predictionapparatus 100 is configured to predict the sample values of theplurality of samples 405 of the current video coding block 401 on thebasis of the plurality of primary reference sample values and theplurality of secondary reference sample values using a planar intraprediction mode.

In an embodiment, the processing unit 101 is configured to predict arespective sample value of a respective sample of the plurality ofsamples 405 of the current video coding block 401 on the basis of aweighted sum of a respective primary reference sample value and arespective secondary reference sample value.

Furthermore, in the weighted sum of a respective primary referencesample value and a respective secondary reference sample value theprocessing unit 101 is configured to weight the respective primaryreference sample value and the respective secondary reference samplevalue as a function of the distance between the respective primaryreference sample and the respective sample, d_(prs), and the distancebetween the respective secondary reference sample and the respectivesample, d_(srs), as illustrated in FIG. 7 and also known as“distance-weighted directional intra prediction” (DWDIP).

In a second implementation form of embodiments of the invention,intra-prediction of the current video coding block 401 can be conductedwithout the step of calculating the gradient signal values at thepositions of the secondary reference samples 601. The detailed steps ofthe second implementation form of the invention, which can be performedby the processing unit 101 of the intra prediction apparatus 100, willbe discussed further below. The reference samples 403 in the followingare referred only to the primary reference samples 403, i.e., the pixelsin the row of pixels above the current video coding block 401 and thepixels in the column of pixels to the left of the current video codingblock 401.

Steps 1 to 4 of the second implementation form are basically similar tothose of the first implementation form discussed above and can besummarized as follows: Firstly, the processing unit 101 of the intraprediction apparatus 100 is configured to estimate parameters of aMultiple Linear Regression (MLR) and fit a plane 501 to the referencesamples 403 (step 1). In steps 2 and 3, the processing unit 101 canoptionally clip the plane (regression) parameters and then use theclipped plane parameters to calculate gradient signal values atpositions of the reference samples 403. In step 4, the processing unit101 is configured to determine for each reference sample 403 arespective reference offset value between the reference sample value andthe corresponding fitting sample value.

Steps 5 to 7 of the second implementation form are essentially distinctfrom those of the first implementation form in intra-predicting thesample values of the plurality of samples 405 of the current videocoding block 401. Steps 5 to 7 will be explained with reference to FIG.8 further below.

FIG. 8 shows an exemplary current video coding block 401, illustratingan aspect of the intra prediction apparatus 100 and the intra predictionmethod 300 according to an embodiment, in particular the relationshipbetween the reference pixels 403 and the plurality of samples 405 of thecurrent video coding block 401 to be intra predicted, wherein the greysquare represents the exemplary currently processed video coding block401 and the squares with dots denote the reference samples 403.

In step 5, the processing unit 101 is configured to determine for arespective sample value of the plurality of samples 405 of the currentvideo coding block 401 a respective sample propagated/interpolatedoffset value on the basis of at least a subset of the plurality ofreference offset values and the selected directional intra predictionmode.

In an embodiment, the processing unit 101 is configured to predict therespective sample propagated/interpolated offset value at each sampleposition on the basis of at least a subset of the plurality of referenceoffset values and the selected directional intra prediction mode on thebasis of an intra prediction mode defined in the HEVC/H.265 standard ora standard evolved therefrom.

In step 6, the processing unit 101 is configured to use the clippedplane parameters to determine the fitting sample value of the respectivefitting sample at each sample position within the block 401 to bepredicted. More specifically, in an embodiment, the processing unit 101is configured to calculate the fitting sample value {circumflex over(p)}_(pred)[x,y] of the respective fitting sample at each sampleposition within the block by the following equation:{circumflex over (p)} _(pred)[x,y]=ax+by+c,wherein x, y denote the position of the fitting sample and parameters a,b and c are the fitting plane parameters.

In step 7, the processing unit 101 is configured to predict or generatethe respective sample value on the basis of the respective samplepropagated/interpolated offset value and the fitting sample value of therespective fitting sample by adding the respective samplepropagated/interpolated offset value (i.e., the result of step 5) to thefitting sample value of the respective fitting sample (i.e., the resultof step 6). More specifically, in an embodiment, the processing unit 101is configured to intra predict the plurality of samples 405 of thecurrent video coding block 401 by the following equation:p _(pred)[x,y]={circumflex over (p)} _(pred)[x,y]+Δ_(pred)[x,y],wherein Δ_(pred)[x,y] denotes the propagated/interpolated offset valueand {circumflex over (p)}_(pred)[x,y] denotes the fitting sample valuefor each sample within the current video coding block 401.

In a further embodiment, the processing unit 101 is configured to applythe DWDIP interpolation mechanism or PDPC (position dependent intraprediction combination) prediction mechanisms immediately to thedifferences Δ[k], i.e. the determined differences Δ[k] and Δ_(pred)[x,y]are processed by DWDIP or PDPC as reference samples and predictedpixels, respectively. For example, in the embodiment disclosed in FIG. 5the propagation step can be replaced by the DWDIP interpolationmechanism that uses Δ[k] as reference samples. In this case, the DWDIPmechanism should be configured to be able to interpolate negativevalues. Similarly, after step 5 of the embodiment shown in FIG. 8, thepropagated differences Δ_(pred)[x,y] can be processed by PDPC. Moredetails about the PDPC mechanism can be found in WO 2017/058635, whichis fully incorporated herein by reference.

While a particular feature or aspect of the disclosure may have beendisclosed with respect to only one of several implementations orembodiments, such a feature or aspect may be combined with one or morefurther features or aspects of the other implementations or embodimentsas may be desired or advantageous for any given or particularapplication. Furthermore, to the extent that the terms “include”,“have”, “with”, or other variants thereof are used in either thedetailed description or the claims, such terms are intended to beinclusive in a manner similar to the term “comprise”. Also, the terms“exemplary”, “for example” and “e.g.” are merely meant as an example,rather than the best or optimal. The terms “coupled” and “connected”,along with derivatives thereof may have been used. It should beunderstood that these terms may have been used to indicate that twoelements cooperate or interact with each other regardless whether theyare in direct physical or electrical contact, or they are not in directcontact with each other.

Although specific aspects have been illustrated and described herein, itwill be appreciated that a variety of alternate and/or equivalentimplementations may be substituted for the specific aspects shown anddescribed without departing from the scope of the present disclosure.This application is intended to cover any adaptations or variations ofthe specific aspects discussed herein.

Although the elements in the following claims are recited in aparticular sequence with corresponding labeling, unless the claimrecitations otherwise imply a particular sequence for implementing someor all of those elements, those elements are not necessarily intended tobe limited to being implemented in that particular sequence.

Many alternatives, modifications, and variations will be apparent tothose skilled in the art in light of the above teachings. Of course,those skilled in the art readily recognize that there are numerousapplications of the invention beyond those described herein. While theinvention has been described with reference to one or more particularembodiments, those skilled in the art recognize that many changes may bemade thereto without departing from the scope of the invention. It istherefore to be understood that within the scope of the appended claimsand their equivalents, the invention may be practiced otherwise than asspecifically described herein.

What is claimed is:
 1. An apparatus for intra prediction of a currentblock of a frame of a video signal, the frame comprising a plurality ofblocks, the plurality of blocks including the current block and a set ofneighboring blocks, the neighboring blocks adjoining the current block,each block comprising a plurality of samples, each sample having aposition and a sample value, the apparatus comprising: a processorconfigured to: determine a fitting plane on a basis of a plurality ofreference samples, the plurality of reference samples comprising samplesof the neighboring blocks, wherein the fitting plane defines, for anyposition within the frame, a respective fitting sample value, andwherein fitting plane parameters of the fitting plane include a, b, andc; and predict, for a selected directional intra prediction mode, samplevalues of a plurality of samples of the current block based on theplurality of reference samples and fitting samples of the fitting plane;wherein for the predicting the sample values of the plurality of samplesof the current block, the processor is configured to: determine, foreach reference sample, a respective reference offset value between areference sample value and a corresponding fitting sample value:determine, for a respective sample value of the plurality of samples ofthe current block, a respective sample propagated/interpolated offsetvalue based on at least a subset of a plurality of reference offsetvalues and the selected directional intra prediction mode; and generatea respective sample value based on the respective samplepropagated/interpolated offset value and the fitting sample value of therespective fitting sample.
 2. The apparatus of claim 1, wherein thefitting plane corresponds to the following equation:{circumflex over (p)} _(pred)[x,y]=ax+by+c, wherein x, y is a positionwithin the frame, {circumflex over (p)}_(pred)[x,y] is a fitting samplevalue, and wherein the processor is further configured to determine thefitting plane parameters a, b and c based on the plurality of referencesamples.
 3. The apparatus of claim 2, wherein the processor is furtherconfigured to perform a multi-linear regression, comprising a leastsquares method, for determining the fitting plane parameters based onthe plurality of reference samples.
 4. The apparatus of claim 3, whereinthe processor is further configured to clip the respective fitting planeparameters a, b and/or c, in case the respective fitting planeparameters a, b and/or c lie outside of respective predefined allowableranges of fitting plane parameters.
 5. The apparatus of claim 1, whereinthe processor is configured to generate the respective sample valuebased on the respective sample propagated/interpolated offset value andthe fitting sample value of the respective fitting sample by adding therespective sample propagated/interpolated offset value to the fittingsample value of the respective fitting sample.
 6. The apparatus of claim1, wherein the processor is configured to determine the respectivesample propagated/interpolated offset value at each sample positionbased on at least the subset of the plurality of reference offset valuesand the selected directional intra prediction mode based on an intraprediction mode defined in the HEVC/H.265 standard or a standard evolvedtherefrom.
 7. The apparatus of claim 1, wherein the set of neighboringblocks of the current block comprises at least one of a video codingblock above the current block and a video coding block to the left ofthe current block.
 8. The apparatus according to claim 1, furthercomprising: an encoding device configured to encode the current block ona basis of a predicted video coding block.
 9. The apparatus according toclaim 1, further comprising: a restoration device configured to restorea video coding block on the basis of an encoded video coding block and apredicted video coding block.
 10. A method for intra prediction of acurrent block of a frame of a video signal, the frame comprising aplurality of blocks, the plurality of blocks including the current blockand a set of neighboring blocks, the neighboring blocks adjoining thecurrent block, each block comprising a plurality of samples, each samplehaving a position and a sample value, the method comprising: determininga fitting plane on a basis of a plurality of reference samples, theplurality of reference samples comprising samples of the neighboringblocks, wherein the fitting plane defines, for any position within theframe, a respective fitting sample value, and wherein fitting planeparameters of the fitting plane include a, b, and c; and predicting, fora selected directional intra prediction mode, sample values of aplurality of samples of the current block based on the plurality ofreference samples and fitting samples of the fitting plane; wherein thepredicting the sample values of the plurality of samples of the currentblock, comprises: determining, for each reference sample, a respectivereference offset value between a reference sample value and acorresponding fitting sample value; determining, for a respective samplevalue of the plurality of samples of the current block, a respectivesample propagated/interpolated offset value based on at least a subsetof a plurality of reference offset values and the selected directionalintra prediction mode; and generating a respective sample value based onthe respective sample propagated/interpolated offset value and thefitting sample value of the respective fitting sample.
 11. Anon-transitory computer-readable medium comprising program code which,when executed by a processor, causes the processor to facilitateexecution of a method for intra prediction of a current block of a frameof a video signal, the frame comprising a plurality of blocks, theplurality of blocks including the current block and a set of neighboringblocks, the neighboring blocks adjoining the current block, each blockcomprising a plurality of samples, each sample having a position and asample value, the method comprising: determining a fitting plane on abasis of a plurality of reference samples, the plurality of referencesamples comprising samples of the neighboring blocks, wherein thefitting plane defines, for any position with the frame, a respectivefitting sample value, and wherein fitting plane parameters of thefitting plane include a, b, and c; predicting, for a selecteddirectional intra prediction mode, sample values of a plurality ofsamples of the current block based on the plurality of reference samplesand fitting samples of the fitting plane; wherein the predicting thesample values of the plurality of samples of the current block,comprises: determining, for each reference sample, a respectivereference offset value between a reference sample value and acorresponding fitting sample value: determining, for a respective samplevalue of the plurality of samples of the current block, a respectivesample propagated/interpolated offset value based on at least a subsetof a plurality of reference offset values and the selected directionalintra prediction mode; and generating a respective sample value based onthe respective sample propagated/interpolated offset value and thefitting sample value of the respective fitting sample.
 12. The method ofclaim 10, wherein the fitting plane corresponds to the followingequation:{circumflex over (p)} _(pred)[x,y]=ax+by+c, wherein x, y is a positionwithin the frame, {circumflex over (p)}_(pred)[x,y] is a fitting samplevalue, and wherein the fitting plane parameters a, b and c aredetermined based on the plurality of reference samples.
 13. The methodof claim 10, further comprising performing a multi-linear regression,comprising a least squares method, for determining the fitting planeparameters based on the plurality of reference samples.
 14. The methodof claim 13, further comprising clipping the respective fitting planeparameters a, b and/or c, in case the respective fitting planeparameters a, b and/or c lie outside of respective predefined allowableranges of fitting plane parameters.
 15. The method of claim 10,comprising generating the respective sample value based on therespective sample propagated/interpolated offset value and the fittingsample value of the respective fitting sample by adding the respectivesample propagated/interpolated offset value to the fitting sample valueof the respective fitting sample.
 16. The method of claim 10, comprisingdetermining the respective sample propagated/interpolated offset valueat each sample position based on at least a subset of the plurality ofreference offset values and the selected directional intra predictionmode based on an intra prediction mode defined in the HEVC/H.265standard or a standard evolved therefrom.
 17. The method of claim 10,wherein the set of neighboring blocks of the current block comprises atleast one of a video coding block above the current block and a videocoding block to the left of the current block.